Method for Preparing Nano-Sized Metal Powder Feedstock and Method for Producing Sintered Body Using the Feedstock

ABSTRACT

A method for preparing a nano-sized metal powder feedstock comprises the steps of preparing a nano-sized metal powder, mixing the metal powder with a solution of an organic binder in a solvent, and wet-milling the mixture so that aggregates of the metal powder are uniformly formed. Further disclosed is a method for producing a sintered body using the feedstock.

TECHNICAL FIELD

The present invention relates to a method for preparing a nano-sizedmetal powder feedstock. More particularly, the present invention relatesto a method for preparing a feedstock suitable for the production of asintered body of a nano-sized metal powder that can be completelycompacted without deformation, such as twists and cracks, and a methodfor producing a sintered body using the feedstock.

BACKGROUND ART

Conventional methods for producing a sintered body using amicrometer-sized metal powder are commonly performed by powder injectionmolding. A molded body of the metal powder formed after removal of anorganic binder has a degree of compaction as low as about 50%, and thuscomplete compaction and uniform shrinkage of the sintered body cannot beachieved.

A process for providing a large driving force for sintering, such ashigh temperature sintering, is required for complete compaction of asintered body. The problems encountered with the process are that largeparticles are grown and thus undesirable modifications tocharacteristics of the raw materials are involved, deteriorating thephysical properties of the sintered body. To solve the above problems,additional processing steps, e.g., addition of a small amount of analloy element, pressurization during sintering, and remolding, have beenemployed.

To improve the mechanical properties of a sintered body of an Fe—Nibased powder, which is the most widely sintered body as a material for apowder metallurgical product, carburization and post-annealing aftersintering are mainly used. However, such additional processing rendersthe overall procedure complex, entails considerable production costs,and causes poor corrosion resistance due to the presence of carbon addedduring carburization annealing. In conclusion, the conventional methodsfor producing a sintered body using a micrometer-sized metal powder havethe problems that the molded body has a low density, the productionprocedure is complicated by the additional processing, and the physicalproperties of the sintered body are deteriorated.

In attempts to basically solve the problems of the conventional methods,methods for producing a sintered body using a nano-sized metal powderhaving a size of 100□ or less are now being actively undertaken. Sincenano-sized metal powders have superior sinterability, they can beuniformly shrunken and completely compacted bylow-temperature/atmospheric pressure sintering techniques. In addition,since nano-sized metal powders have highly uniform and fine crystallinestructures, product characteristics are improved. Under thesecircumstances, application of nano-sized metal powders to metalinjection molding techniques is actively under study.

However, despite such superior availability of nano-sized metal powders,production and sintering compaction techniques have not been establishedand thus application to the production of near-net sintered bodies is asyet insufficient.

Korean Patent No. 0366773 (Title: A method for producing a nano-sizedmetal powder feedstock for metal injection molding, patentee: HanyangEducational Institute) suggests a method for producing a feedstock formetal injection molding by which explosive oxidation of the nano-sizedmetal powder can be controlled and complete compaction of a product canbe achieved while maintaining the shape of the product duringproduction. According to this method, the coating of a binder to thenano-sized metal powder inhibits explosive oxidation of the nano-sizedmetal powder and improves complete compaction of the product.

However, the method is limited to the production of a feedstock of thenano-sized metal powder, and fails to sufficiently consider theapplicability to a near-net product. Specifically, since the nano-sizedpowder has a large interfacial energy, non-uniform pore distribution mayarise inside the molded body after debinding. In addition, pores remaineven after low-temperature/atmospheric pressure sintering, thusdeteriorating the mechanical properties of a sintered body. Accordingly,the nano-sized powder should undergo high-temperature sintering at above1,000° C. However, high-temperature sintering causes deterioratedphysical properties of the sintered body, and makes it impossible toutilize the advantages of low-temperature/atmospheric pressuresintering, i.e. complete compaction and growth of particles. Inaddition, according to the method, five or six thermoplastic bindershaving different debinding temperatures are used in order to prevent thedeformation of the molded body arising from rapid removal of the binder,which occupies 40˜60% of the total volume, during debinding.Accordingly, the method has problems of complicated procedure andincreased production costs. Further, since an elevation in debindingtemperature should be sufficiently slow, the overall processing time islengthened.

Thus, there is a need in the art for a method for preparing a nano-sizedmetal powder feedstock practically applicable to the production of anear-net sintered body and suitable for low-temperature/atmosphericpressure sintering, and a method for producing a sintered body using thefeedstock.

DISCLOSURE OF INVENTION Technical Problem

Therefore, the present invention has been made in view of the aboveproblems of the prior art, and it is an object of the present inventionto provide a method for preparing a nano-sized metal powder feedstocksuitable for the production of a sintered body that can be completelycompacted by preventing the occurrence of coarse pores during subsequentdebinding through the structural control of the nano-sized metal powder.

It is another object of the present invention to provide a method forproducing a sintered body that is completely compacted and has a uniformgrain size by debinding for a shorter period of time using thefeedstock.

Technical Solution

In accordance with one aspect of the present invention for achieving theabove objects, there is provided a method for preparing a nano-sizedmetal powder feedstock comprising the steps of preparing a nano-sizedmetal powder, mixing the metal powder with a solution of an organicbinder in a solvent, and wet-milling the mixture so that aggregates ofthe metal powder are uniformly formed.

Preferably, the mixing step and the wet-milling step are simultaneouslycarried out to simplify the procedure of the method.

According to the method of the present invention, since pores can beuniformly distributed during the subsequent formation of a molded body,desired debinding can be carried out without deformation of the moldedbody, despite mixing of only one or two organic binders with the metalpowder.

It is preferred that the organic binder is a water-soluble binder andthe solvent is distilled water or alcohol. The water-soluble organicbinder may be stearic acid.

For improved coating effects, the viscosity of the binder solution ispreferably 2 Pa□s or lower at 100˜200° C., and more preferably 1 Pa□s orlower. For sufficient coating effects, there can be preferably used abinder solution having a viscosity of 0.002 Pa□s.

The nano-sized metal powder is an Fe-based alloy powder and contains atleast one metal selected from the group consisting of Ni, Cu, Mo and W.A representative nano-sized metal powder is an Fe—Ni powder whose Nicontent is 2˜80 wt %.

The mixing step may further include the sub-step of adding a surfactantto the mixture. At this step, the surfactant is preferably added in anamount not exceeding 2 wt %.

The mixing step and the wet-milling step are preferably carried out in astate where atmospheric air is blocked. Specifically, the steps can becarried out in an inert gas or protective gas atmosphere.

In accordance with another aspect of the present invention, there isprovided a method for producing a sintered body using a nano-sized metalpowder. The method comprises the steps of preparing the nano-sized metalpowder feedstock, molding the nano-sized metal powder feedstock into adesired shape, debinding the molded body, and sintering the deboundbody.

In a specific embodiment, the molding step can be carried out byinjection molding or extrusion molding. The debinding step is carriedout by heating the molded body to about 300° C. to about 500° C. at arate 3˜10° C./min., thus shortening the debinding time to about 2 hours.

The sintering step can be carried out by rapidly heating the deboundbody to about 500˜1,000° C. at a rate of 300° C./min. or higher. It ispreferred that the sintering step is carried out consecutively after thedebinding step.

The sintered body thus produced has a grain size of 200□ or less and adegree of compaction of 95% or higher.

Hereinafter, various features of the present invention and effectsthereof will be explained in more detail.

The present invention is characterized in that the size of theaggregates of the nano-sized metal powder is uniformly controlled sothat the aggregates can be applied to low-temperature/atmosphericpressure sintering. Specifically, in the method for preparing anano-sized metal powder feedstock according to the present invention,the nano-sized metal powder is mixed with the organic binder in asolution state and is wet-milled, thereby maintaining the size of theaggregates at a uniform level.

The use of the binder solution allows the binder to be more effectivelycoated on the surface of the powder particles so that oxidation of theparticles is prevented. Accordingly, even when a small amount of thebinder is added during molding, the viscosity of the binder solution islowered for sufficient coating, thus providing a nano-sized metal powderfeedstock that can be stored in air for a prolonged period of timewithout oxidative contamination.

The binder is commonly mixed in an amount of from about 2% to about 50%.In the case where the binder is added in a relatively small amount,e.g., in bi-directional compression molding, sufficient coating effectscannot be attained by the method disclosed in Korean Patent No. 0366773.In contrast, the use of the binder solution and wet-milling process inthe present invention ensures uniform distribution of the aggregates andmore effective coating of the binder.

The solvent used to form the binder solution is not especially limitedto distilled water or alcohol. Any solvent can be used so long as itforms a binder solution having a sufficiently low viscosity. Variousknown solvents can be used depending on the particular kind of thebinder. At this time, the binder solution preferably has a viscosity nothigher than about 2 Pa□s at from about 100° C. to 200° C.

In the method for preparing a feedstock of the present invention, thestep of mixing the nano-sized metal powder and the binder solution andthe wet-milling step for uniform size control of the aggregates can besimultaneously carried out to simplify the procedure of the method. Forexample, the binder solution and the nano-sized metal powder are chargedinto a milling machine and the mixture is milled. Mixing and grinding inthe milling machine enables both the coating of the binder solution andsize control of the aggregates. These processing steps are preferablycarried out in a state where atmospheric air is blocked. Morespecifically, it is preferred that the processing steps are carried outin clean equipment filled with an inert or protective gas.

For better uniform distribution, a small amount of a surfactant can beoptionally used as a dispersant. The surfactant is preferably added inan amount not exceeding 2 wt %, based on the weight of the mixture, soas not to deteriorate the characteristics of the sintered body. Forsufficient effects of the surfactant, it is preferred to add thesurfactant in an amount of 0.5 wt % or higher.

In the case where the nano-sized metal feedstock is used to produce asintered body, aggregates are uniformly distributed in the feedstock andthus occurrence of coarse pores is prevented. Accordingly, deformationarising from separation of the binder can be minimized during thesubsequent debinding. Accordingly, unlike in conventional methods (wherefive or more binders are used depending on temperature gradients), oneor two binders can be used in the present invention, thus simplifyingthe procedure of the method. In addition, the debinding is conducted byheating the molded body to 300˜500° C. at a rate 3˜10° C./min., thusshortening the debinding time to about 2 hours.

Furthermore, since the debound body has a uniform particle size withoutoccurrence of coarse pores, low-temperature/atmospheric pressuresintering at a temperature range of 500˜1,000° C. can be applied to thedebound body to manufacture a nano-sized metal product having a grainsize of 200□ or less and a degree of compaction of 95% or higher.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a scanning electron micrograph (SEM) of a nano-sized Fe—Nialloy powder that can be used in the present invention.

FIG. 2 is a graph showing the results of the particle size analysis of anano-sized Fe—Ni alloy powder feedstock prepared in Example 1 of thepresent invention using a laser particle size analyzer.

FIGS. 3 and 4 are scanning electron micrographs of the broken side of adebound body of a nano-sized Fe—Ni alloy powder obtained in Example 2 ofthe present invention at different magnifications (200× and 20,000×,respectively).

FIG. 5 shows photographs of a molded body and a sintered body of anano-sized Fe—Ni alloy powder produced in Example 2 of the presentinvention.

FIG. 6 is an optical micrograph (200×) of a sintered body of anano-sized Fe—Ni alloy powder produced in Example 2 of the presentinvention.

FIG. 7 is a scanning electron micrograph of an overetched surface of thesintered body shown in FIG. 6.

MODE FOR THE INVENTION

Preferred embodiments of the present invention will be described in moredetail with reference to the accompanying drawings. The advantages andeffects of the present invention will be better understood by theembodiments.

Example 1

First, a nano-sized Fe—Ni alloy powder was prepared as a nano-sizedmetal powder in accordance with the following procedure. Specifically,an Fe oxide and an Ni oxide, each of which had an average particle sizeof 1 m, were mixed together to have a weight ratio of 92:8, and werethen subjected to high-energy ball milling in a steel attritor for 10hours to finely pulverize the mixture to a size of 10˜20□.

Thereafter, the pulverized mixture was dried and reduced under ahydrogen atmosphere at 450° C. for 40 minutes to prepare a nano-sizedFe-8 wt % Ni alloy powder. As shown in FIG. 1, particles having a sizeof about 70□ gathered to form aggregates having a size of about 5 m totens of micrometers.

Next, a binder solution and a surfactant were added to the nano-sizedFe-8 wt % Ni alloy powder. The binder solution was prepared by mixing 5g of stearic acid (CH₃(CH₂)COOH) and 35 ml of ethanol as a solvent. Asthe surfactant, 0.5 mol of octanol (C₈H₁₈O) was used.

In this example, the mixing was conducted together with wet-millingusing a three-dimensional mixer. Specifically, the milling was conductedusing 40 g of steel balls at 60 rpm for 9 hours. The milled mixture wasdried until the loading rate of the nano-sized Fe-8 wt % Ni alloy powderreached 50%, to prepare a nano-sized metal powder feedstock.

FIG. 2 is a graph showing the results of the particle size analysis ofthe nano-sized Fe—Ni alloy powder feedstock using a laser particle sizeanalyzer (LPA). As described above, the nano-sized Fe—Ni alloy powderfeedstock was prepared by adding 0.5 mol of octanol as a surfactant tothe Fe-8 wt % Ni nano-sized metal powder in 35 ml of ethyl alcohol, andwet-milling the mixture using 40 g of steel balls for 9 hours. The laserparticle size analysis indicates that the powder particles with a sizeof tens of micrometers were efficiently pulverized and dispersed bywet-milling to form aggregates with an average size of 700□.

Example 2

In this example, a cylindrical sintered body was produced using anano-sized metal powder feedstock.

First, the nano-sized metal powder feedstock prepared in Example 1 wasinjected into a cylindrical mold under 100 MPa at 100° C. to produce acylindrical molded body. The cylindrical molded body thus produced wascompacted to about 52% (see the left hand side of FIG. 5).

Thereafter, to protect the injection-molded nano-sized Fe-8 wt % Nialloy powder against occurrence of cracks by oxidation, the molded bodywas subjected to debinding by heating to 400° C. at a rate of 5° C./min.

FIG. 3 is a scanning electron micrograph (200×) of the broken side ofthe sample obtained after debinding, and FIG. 4 is a scanning electronmicrograph (20,000×) of the broken side of the debound body. As shown inFIGS. 3 and 4, no coarse pores (micrometer-scale pores) adverselyaffecting the subsequent sintering process were observed, and instead, afine structure consisting of uniform particles with a size not largerthan 100□ was observed. This is because the aggregates wet-milled inExample 1 were uniformly filled into pores between unpulverizedaggregates.

Based on the uniform distribution of the aggregates, only one bindercould be used to prevent the occurrence of coarse pores arising fromseparation of the binder. Unlike in conventional methods where five ormore binders are used, the debinding time could be shortened to 2 hours.

Subsequently to the debinding, the debound body was heated to 700° C. ata rate of 300° C./min., and was sintered for from 30 minutes to 4 hoursto produce a cylindrical sintered body having a degree of compaction of95% (see the right hand side of FIG. 5).

Compared to the cylindrical molded body having a degree of compaction of52% shown in FIG. 5, no deformation, such as twists and cracks, wasobserved in the cylindrical sintered body even after debinding andsintering, and the shape of the cylindrical sintered body was unchangedduring molding. As is evident from this example, the sintered bodyhaving a degree of compaction of 95% or higher could be produced fromthe debound body having a degree of compaction of 52% (after debinding)even at a sintering temperature as low as 700° C. Therefore, thesintered body can be useful in the manufacture of a complicated near-netsintered product.

FIG. 6 is an optical micrograph (200×) showing the fine structure of thecylindrical Fe-8 wt % Ni sintered body. As shown in FIG. 6, the sinteredbody has a completely compacted structure (degree of compaction: 95% orhigher). FIG. 7 is a scanning electron micrograph (5,000×) showing thegrain of the cylindrical Fe-8 wt % Ni sintered body after overetching.As shown in FIG. 7, the sintered body has a fine structure wherein thegrains having a size of about 300□ are uniformly distributed.

To evaluate the mechanical properties of the Fe-8 wt % Ni sintered body,the micro-hardness values at ten or more sites of the sintered body weremeasured with a load of 200 g by means of a micro Vickers hardnesstester, and averaged. The obtained average value was compared with thestandard hardness values of commercially available injection moldedsintered bodies. The results are shown in Table 1 below.

TABLE 1 Sintered body Composition Hardness (Hv) Example 2 Fe-8 wt % Ni298 MIM-2200 Fe-2 wt % Ni 85 MIM-2700 Fe-7 wt % Ni 130 MIM-4605 Fe-2 wt% Ni-0.5 wt % C 110 MIM-Fe2Ni Fe-2 wt % Ni-0.6 wt % C 300 MIM-Fe8Ni Fe-8wt % Ni-0.6 wt % C 340

The sintered bodies (MIM-2200, MIM-2700 and MIM-4605) according to thestandard specification adopted by the Metal Powder Industry Federation(MPIF) have a Vickers hardness of 85˜130. In addition, since theinjection molded metal powder sintered bodies according to the Japanesestandard specification were subjected to carburization and annealing inorder to improve the mechanical properties of the injection molded Fe—Nipowders, they have a high Vickers hardness of 300 (in the case of 2 wt %Ni) and 340 (in the case of 8 wt % Ni). The Fe-8 wt % Ni sintered bodyproduced in the present invention has a Vickers hardness of 298, whichis greater than two times that specified in the U.S standardspecification. In addition, the Vickers hardness of the Fe-8 wt % Nisintered body produced in the present invention is comparable to thatspecified in Japanese standard specification without involvingadditional carburization and annealing for improving the mechanicalproperties of the sintered body.

Although the present invention has been described herein with referenceto the foregoing embodiments and the accompanying drawings, the scope ofthe invention is defined by the claims that follow. Accordingly, thoseskilled in the art will appreciate that various substitutions,modifications and changes are possible, without departing from thetechnical spirit of the present invention as disclosed in theaccompanying claims, and such substitutions, modifications and changesare within the scope of the present invention.

INDUSTRIAL APPLICABILITY

As apparent from the foregoing, according to the method of the presentinvention, the application of wet-milling in the presence of the bindersolution allows the binder to be more effectively coated on the surfaceof the powder particles and enables uniform control of the size of theaggregates. Since the nano-sized metal powder feedstock prepared by themethods of the present invention can maintain the internal structure ofthe molded body uniform and fine, completely compacted near-netnanostructured products can be manufactured without deformation, such astwists and cracks, even after sintering using the nano-sized metalpowder feedstock. Therefore, according to the method of the presentinvention, simplification of the production procedure and reduction inproduction costs can be expected.

1. A method for preparing a nano-sized metal powder feedstock,comprising the steps of: preparing a nano-sized metal powder; mixing themetal powder with a solution of an organic binder in a solvent; andwet-milling the mixture so that aggregates of the metal powder areuniformly formed.
 2. The method according to claim 1, wherein the mixingstep and the wet-milling step are simultaneously carried out.
 3. Themethod according to claim 1, wherein the metal powder is mixed with oneor two organic binders.
 4. The method according to claim 1, wherein theorganic binder is a water-soluble binder and the solvent is distilledwater or alcohol.
 5. The method according to claim 4, wherein thewater-soluble organic binder is stearic acid.
 6. The method according toclaim 1, wherein the binder solution has a viscosity of 2 Pa·s or lowerat 100˜200° C.
 7. The method according to claim 1, wherein thenano-sized metal powder is an Fe-based alloy powder and contains atleast one metal selected from the group consisting of Ni, Cu, Mo and W.8. The method according to claim 7, wherein the nano-sized metal powderis an Fe—Ni powder whose Ni content is 2˜80 wt %.
 9. The methodaccording to claim 1, wherein the mixing step further includes thesub-step of adding a surfactant to the mixture.
 10. The method accordingto claim 9, wherein the surfactant is added in an amount not exceeding 2wt %.
 11. The method according to claim 1, wherein the mixing step andthe wet-milling step are carried out in a state where atmospheric air isblocked.
 12. The method according to claim 11, wherein the mixing stepand the wet-milling step are carried out in an inert gas or protectivegas atmosphere.
 13. A method for producing a sintered body using anano-sized metal powder, comprising the steps of: preparing a nano-sizedmetal powder feedstock prepared by the method according to claim 1;molding the nano-sized metal powder feedstock into a desired shape;debinding the molded body; and sintering the debound body.
 14. Themethod according to claim 13, where the molding step is carried out byinjection molding or extrusion molding.
 15. The method according toclaim 14, where the debinding step is carried out by heating the moldedbody to about 300° C. to about 500° C. at a rate 3˜10° C./min.
 16. Themethod according to claim 14, where the sintering step is carried out bysintering the debound body at 500˜1,000° C.
 17. The method according toclaim 16, where the sintered body has a grain size of 200 nm or less anda degree of compaction of 95% or higher.
 18. A method for producing asintered body using a nano-sized metal powder, comprising the steps of:preparing a nano-sized metal powder feedstock prepared by the methodaccording to claim 7; molding the nano-sized metal powder feedstock intoa desired shape; debinding the molded body; and sintering the deboundbody.
 19. A method for producing a sintered body using a nano-sizedmetal powder, comprising the steps of: preparing a nano-sized metalpowder feedstock prepared by the method according to claim 8; moldingthe nano-sized metal powder feedstock into a desired shape; debindingthe molded body; and sintering the debound body.
 20. A method forproducing a sintered body using a nano-sized metal powder, comprisingthe steps of: preparing a nano-sized metal powder feedstock prepared bythe method according to claim 12; molding the nano-sized metal powderfeedstock into a desired shape; debinding the molded body; and sinteringthe debound body.